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Brainfuck Just-In-Time Compiler

Introduction

Brainfuck is a very interesting programming language that has only 8 operators:

Operator Code in C/C++
+ buff[ptr]++
- buff[ptr]--
> ptr++
< ptr--
[ if(!buff[ptr]) goto ']'
] if(buff[ptr] goto '['
, buff[ptr]=getchar()
. putchar(buff[ptr])

This simple syntax makes brainfuck a great language for me to learn how to build an interpreter and jit(just-in-time) compiler.

Brainfuck Interpreter

This project has a simple interpreter for brainfuck, using switch-threading:

for(size_t i=0;i<code.size();++i){
    switch(code[i].op){
        case op_add:buff[p]+=code[i].num;break;
        case op_sub:buff[p]-=code[i].num;break;
        case op_addp:p+=code[i].num;break;
        case op_subp:p-=code[i].num;break;
        case op_jt:if(buff[p])i=code[i].num;break;
        case op_jf:if(!buff[p])i=code[i].num;break;
        case op_in:buff[p]=getchar();break;
        case op_out:putchar(buff[p]);break;
    }
}

To optimize the efficiency of the interpreter, i count consecutive operators instead of just translating operators into an opcode. You will see the structure of opcode in jit.cpp.

For example:

bf code opcode
+++ buf[p]+=3
---- buf[p]-=4
>>>>> p+=5
<< p-=2

Just-In-Time Compiler

mmap

After generating opcodes, it's quite easy for us to generate machine codes into a memory space allocated by mmap.

This memory space must be read/write/exec so we could execute the machine codes in this memory space. You could see the mmap in amd64jit::amd64jit(const size_t) in file amd64jit.h.

I use a global u8 array buff[0x20000] to be the paper of brainfuck machine(and rbx stores the pointer), and remember to use memset to clean the stack space to zero-filled.

/* set bf machine's paper pointer */
mem.push({0x48,0xbb}).push64((uint64_t)buff); // movq $buff,%rbx

Add & Sub Operations

These four operators are not so difficult to translate to machine codes:

case op_add:  mem.push({0x80,0x03,(uint8_t)(op.num&0xff)}); break; // addb $op.num,(%rbx)
case op_sub:  mem.push({0x80,0x2b,(uint8_t)(op.num&0xff)}); break; // subb $op.num,(%rbx)
case op_addp: mem.push({0x48,0x81,0xc3}).push32(op.num);    break; // add $op.num,%rbx
case op_subp: mem.push({0x48,0x81,0xeb}).push32(op.num);    break; // sub $op.num,%rbx

Library Function putchar & getchar

putchar

int putchar(int);

op_out uses the putchar, write a demo and use objdump to see how the gcc and clang generate the machine code that calls the function, then just copy them :)

mem.push({0x48,0xb8}).push64((uint64_t)putchar); // movabs $putchar,%rax
#ifndef _WIN32
mem.push({0x0f,0xbe,0x3b}); // movsbl (%rbx),%edi
#else
mem.push({0x0f,0xbe,0x0b}); // movsbl (%rbx),%ecx
#endif
mem.push({0xff,0xd0}); // callq *%rax

You may find that there's a small difference between generated machine code on Windows platform. This is because the rule of parameter passing in call convention of Windows is different from Linux/macOS/Unix. And Linux/macOS/Unix use rdi to get the first parameter, but Windows uses rcx.

Although JIT-compiler developers should remember this rule, it is quite easier to remember x86_64/amd64 call convention than x86_32...

getchar

int getchar();

op_in uses the getchar, also we just use the objdump to see how gcc/clang generate the code, and just copy them :)

Luckily, on Windows/Linux/macOS/Unix platform, the return value int will all be stored in register rax. And we just need to mov the low 8-bits of rax to rbx[0] (aka movsbl %al,(%rbx)).

mem.push({0x48,0xb8}).push64((uint64_t)getchar); // movabs $getchar,%rax
mem.push({0xff,0xd0}); // callq *%rax
mem.push({0x88,0x03}); // movsbl %al,(%rbx)

So we don't need to write #ifndef _WIN32 and so on :)

Jump Operation

je and jne are two difficulties in this project. You must calculate the distance of two jump labels to make sure they work correctly.

amd64jit& amd64jit::je() {
    push({0x0f,0x84}).push32(0x0);// je
    stk.push(ptr);
    return *this;
}

amd64jit& amd64jit::jne() {
    push({0x0f,0x85}).push32(0x0);// jne
    uint8_t* je_next=stk.top();stk.pop();
    uint8_t* jne_next=ptr;
    uint64_t p0=jne_next-je_next;
    uint64_t p1=je_next-jne_next;
    jne_next[-4]=(p1&0xff);
    jne_next[-3]=((p1>>8)&0xff);
    jne_next[-2]=((p1>>16)&0xff);
    jne_next[-1]=((p1>>24)&0xff);
    je_next[-4]=(p0&0xff);
    je_next[-3]=((p0>>8)&0xff);
    je_next[-2]=((p0>>16)&0xff);
    je_next[-1]=((p0>>24)&0xff);
    return *this;
}

op_jf([) uses the je and op_jt(]) uses the jne.

Conclusion

bf code opcode machine code
+ op_add addb $op.num,(%rbx)
- op_sub subb $op.num,(%rbx)
> op_addp add $op.num %rbx
< op_subp sub $op.num %rbx
[ op_jf je label
] op_jt jne label
, op_in callq *%rax & movsbl %al,(%rbx)
. op_out callq *%rax

Hope you enjoy it.

More

Want to check the output machine code of different CPU arch?

You may need this website:

gcc.godbolt.org